A DC-to-DC converter switches a high side switch and a low side switch connected in series to generate an output current flowing through an inductor to charge a capacitor to generate an output voltage. To regulate the output voltage, the output current is sensed to modulate the duties of the high side and low side switches. To sense the output current, it is employed the virtual ground characteristic of an operational amplifier, for example in U.S. Pat. No. 6,246,220 issued to Isham et al., or the voltage drop of a sense resistor connected in series with the inductor, for example in product no. ISL6565B of Intersil Corporation, or the conductive resistance of the MOS transistor serving as the low side switch, for example in product no. ISL6563 of Intersil Corporation.
In the circuit of U.S. Pat. No. 6,246,220, a virtually grounded operational amplifier senses the output current to generate a signal for the system control of a converter, while due to the virtually grounded operational amplifier and the negative feedback in the system, the sensing speed is slower than that by using an open loop control. If the slew rate of the operational amplifier is increased to improve the sensing speed, it will be easy to have a greater damping in each sensing operation and result in the operational amplifier poorer stability and the sensed value improper.
In the sensing by using a sense resistor connected in series with the inductor, either the parasitic resistor of the inductor or an external resistor, small resistance is hard to accomplish precise detection, and large resistance consumes much power. Alternatively, the conductive resistance of the low side MOS transistor serving as the sense resistor may sense the current more precisely, while the conductive resistance of the low side MOS transistor is temperature dependent, and error is introduced by the varying resistance once the MOS transistor is operated.
Therefore, it is desired a temperature independent current sense circuit and method for a DC-to-DC converter.